The present invention relates to a driver circuit capable of generating an output with a large amplitude from a low power-supply voltage, and also relates to an optical-transmission module employing the driver circuit stated above.
FIG. 14 shows an example of an optical network studied by the inventors in developing the present invention.
The optical network of FIG. 14 uses various optical fibers which send signals at different signal speeds such as 50 Mbps, 600 Mbps, 2.5 Gbps and 10 Gbps. Devices 52 such as transmitting devices and routers employed in the optical network have optical transmitting modules 46, optical receiving modules 53, and optical modules 54 which effectively combine the optical transmitting modules 46 and the optical receiving modules 53, which modules handle signals at their signal speed.
The optical transmitting modules 46, as shown in FIG. 15, can be electrically connected to logic circuits such as a MUX circuit 55 for making low-speed signals time division multiplexed, a header adding circuit 56 for inserting a header, which includes necessary information for the network such as information for error detection, to data, and a scrambling circuit 57 for protecting data from continuing the same data xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d for a long period of time. The logic circuits such as the MUX circuit 55, the header adding circuit 56, and the scrambling circuit 57 are applied with a low power-supply voltage such as 5V or xe2x88x925.2xcx9c3.3V for making them low power consumption devices.
A driver circuit employing an optical transmitting module 46 for transmitting signals at low speed less than 600 Mbps comprises a changeable-current supply and current mirror circuit. And it can work with a low power-supply voltage of 3.3V. Such circuits are described in the MAXIM company catalog MAX3667 and are especially shown in FIG. 1 of page 6 of this catalog.
On the other hand, as described on pages 31 to 36 of a technology-research report ICD95-74 of the Institute of Electronics and Communication Engineers of Japan written in the year of 1995, and in particular, as shown in FIG. 8 on page 34 of this thesis, a driver circuit employing an optical transmitting module such as module 46 for transmitting signals at high speed more than 2.5 Gbps is fabricated by adopting a GaAs process to increase the processing speed of the driver circuit. The driver circuit employs a pair of transistors with the emitters thereof connected to each other to function as a differential-amplifier circuit and a constant-current supply circuit connected to the emitter connection point of the pair of transistors. Such a driver circuit 10 is shown in FIG. 12. As shown in the figure, the driver circuit 10 comprises a pair of transistors 11 and 12 forming a differential-amplifier circuit. The transistors 11 and 12 are each a FET (field effect transistor) made of GaAs. The emitters of the transistors are connected to each other. A constant-current supply circuit 13 is provided between the emitter connection point of the transistors 11 and 12 and a power supply 8. The constant-current supply circuit 13 comprises a transistor 15, a resistor 16 and a voltage supply 14. The driver circuit 10 drives a load 2.
However, the related art technology described above, and studied by the inventors, has the following problems.
The driver circuit which comprises a changeable-current supply and a current returning circuit needs a complementary bipolar process for making PNP transistor as a vertical structure and a NPN transistor as a vertical structure on the same chip. Therefore, since it makes the chip structure complicated, the performance of the transistor is inferior to a transistor made by a bipolar process, not a complementary bipolar process. Therefore, this driver circuit can""t employ the optical transmitting module for transmitting signals at high speed.
The GaAs FETs employed in the driver circuit which has the optical transmitting module for transmitting signals at high speed are expensive in comparison with a bipolar transistor made of a material such as Si. By merely replacing the pair of FETs employed in the related art driver circuit with low-cost bipolar transistors made of a material such as Si, however, it is difficult to operate the driver circuit without causing the bipolar transistors to enter a saturated state at a low power-supply voltage such as 3.3V since a voltage in the range of 0.7V to 0.8V is required between the base and the emitter of each of the transistors.
Therefore, since it is difficult to apply the low power-supply voltage to the optical module for transmitting signals at high speed more than 2.5 Gbps, it is also difficult to make the power-supply voltage of the device 52 such as transmitting devices and routers low voltage.
In addition, since two power supplies are needed, one power supply for the logic circuit and the optical module for transmitting signals at low speed less than 600 Mbps and another power supply for the optical module for transmitting signals at high speed more than 2.5 Gbps, the device size become large.
It is thus an object of the present invention to provide a low-cost and high-speed driver circuit capable of generating an output current signal or an output voltage signal with a desired amplitude by using a pair of non-complementary bipolar transistors made of a low-cost material such as Si without causing the bipolar transistors to enter a saturated state at a low power-supply voltage such as 3.3V.
It is another object of the present invention to provide an optical-transmission module capable of transmitting data by driving a laser diode or an optical modulator to generate an optical-transmission wave from an optical-transmission signal and operable with the same low power-supply voltage of the logic circuit and the optical module for transmitting signals at low speed less than 600 Mbps by using the low-cost driver circuit operable at a low power-supply voltage such as 3.3V.
In order to achieve the objects described above, a driver circuit is provided comprising a first differential-amplifier circuit having a pair of first transistors with emitters thereof connected to each other and a first resistor provided between an emitter connection point of the first transistors and first power supply. A difference in electric potential is applied between bases of the pair of the first transistors to set a ratio of a current flowing through one of the pair of the first transistors to a current flowing through the other first transistor at about {fraction (1/100)} or smaller, and an amplitude of an output current is controlled by a higher electric potential applied to the base of one of the first transistors.
Another driver circuit in accordance with the present invention comprises a first differential-amplifier circuit composed of a pair of first transistors employing bipolar transistors with emitters thereof connected to each other and a first resistor with resistance in the range of about 2 to 20 xcexa9 provided between an emitter connection point of the pair of the first transistors and a first power supply, wherein an amplitude of an output current is controlled by electric potential.applied to a base of the first transistor.
An optical-transmission module in accordance with the present invention comprises a driver circuit having a first differential-amplifier circuit, the first differential-amplifier circuit having a pair of first transistors with emitters thereof connected to each other and a first resistor provided between an emitter connection point of the first transistors and a first power supply, with a difference in electric potential applied between bases of the pair of the first transistors to set a ratio of a current flowing through one of the pair of the first transistors to a current flowing through the other first transistor at about {fraction (1/100)} or smaller, and a laser-diode module for outputting an optical-transmission wave by carrying out optical-intensity modulation on a laser diode on a basis of an output current signal or an output voltage signal output by the first differential-amplifier circuit, wherein an output current of the driver circuit is controlled by a higher electric potential applied to the base of one of the first transistors.
Another optical-transmission module in accordance with the present invention comprises a driver circuit having a first differential-amplifier circuit, the first differential-amplifier circuit having a pair of first transistors with emitters thereof connected to each other and a first resistor with resistance in the range of about 2 to 20 xcexa9 provided between an emitter connection point of the pair of the first transistors and a first power supply, and a laser-diode module for outputting an optical-transmission wave by carrying out optical-intensity modulation on a laser diode on a basis of an output current signal or an output voltage signal output by the first differential-amplifier circuit, wherein an output current of the driver circuit is controlled by electric potential applied to a base of the first transistor.
Another optical-transmission module in accordance with the present invention comprises a driver circuit having a first differential-amplifier circuit, the first differential-amplifier circuit having a pair of first transistors with emitters thereof connected to each other and a first resistor provided between an emitter connection point of the first transistors and a first power supply, with a difference in electric potential applied between bases of the pair of the first transistors to set a ratio of a current flowing through one of the pair of the first transistors to a current flowing through the other first transistor at about {fraction (1/100)} or smaller, and an optical-modulation module for outputting an optical-transmission wave by modulating a light emitted by a light source on the basis of an output current signal or an output voltage signal output by said first differential-amplifier circuit, wherein an output current of the driver circuit is controlled by a higher electric potential applied to the base of one of the first transistors.
Another optical-transmission module in accordance with the present invention comprises a driver circuit having a first differential-amplifier circuit, the first differential-amplifier circuit having a pair of first transistors with emitters thereof connected to each other and a first resistor with resistance in the range of about 2 to 20 xcexa9 provided between an emitter connection point of the pair of the first transistors and a first power supply, and a optical-modulation module for outputting an optical-transmission wave by modulating a light emitted by a light source on the basis of an output current signal or an output voltage signal output by said first differential-amplifier circuit, wherein an output current of the driver circuit is controlled by electric potential applied to a base of the first transistor.
Another driver circuit in accordance with the present invention comprises a differential-amplifier circuit having a pair of transistors, means for setting a ratio of a current flowing through one of a pair of transistors to a current flowing through the other transistor at about {fraction (1/100)} or smaller, and means for controlling the amplitude of an output current by the higher electric potential applied to said base of one of said transistors.
Another optical-transmission module in accordance with the present invention comprises a first optical module to transmit signals at low speed, a second optical module to transmit signals at high speed, and a power supply to supply substantially the same power supply voltage to both the first optical module and the second optical module.
Another optical-transmission module in accordance with the present invention comprises a driver circuit having a differential-amplifier circuit with a pair of transistors and means for setting a ratio of a current flowing through one of a pair of transistors to a current flowing through the other transistor at about {fraction (1/100)} or smaller and means for controlling the amplitude of an output current by the higher electric potential applied to said base of one of said transistors, and a optical-modulation module for outputting an optical-transmission wave by modulating a light emitted by a light source on the basis of an output current signal or an output voltage signal output by said differential-amplifier circuit, wherein an output current of the driver circuit is controlled by electric potential applied to a base of the transistor.
As described above, in the driver circuits with the configurations explained above, the amplitude of an output can be increased by a difference in voltage between the base and the emitter of a transistor. As a result, it is possible to employ bipolar transistors made of a low-cost material such as Si in a driver circuit having the amplitude of an output signal at least equal to that of a driver circuit having a differential-amplifier circuit employing a pair of FETs each made of GaAs and the related art constant-current supply.
In addition, in optical-transmission modules having configurations described above, it is possible to transmit data by driving a laser diode or an optical modulator by means of the low-cost driver circuit described above to generate an optical-transmission wave with an improved quality.